1. Field of the Invention
The present invention is directed to a method and apparatus for detecting a state of imminent cardiac arrhythmia, wherein detection of the state of imminent arrhythmia is made by using nerve signals from the autonomic nerves innervating the heart, and for administering appropriate anti-arrhythmia therapy upon detection of the state of imminent cardiac arrhythmia.
2. Description of the Prior Art
In the control of a device for heart therapy, such as a pacemaker, it is known to use signals providing a measure of the body's work load, in addition to utilization of the parameters in the ECG signal generated by the heart itself. These signals can be obtained when one or more load-related physiological variables, such as pH, oxygen saturation in blood etc., is/are detected with sensors. In more advanced devices for heart therapy, in which the device is able to provide many kinds of treatment depending on the condition of the heart, control of the device can also be exercised by utilization of other signals indicative of whether such conditions are either present, or are in the process of becoming established (incipient). Signals of these kinds can be related to hemodynamic conditions, e.g. blood pressure in the right ventricle. A sudden drop in pressure, combined with a very fast heart rate, could be indicative of, e.g., fibrillation in the heart.
In particular, control can be exercised through signals containing information related to the autonomic nervous system (ANS). In addition to being indicative of established heart conditions, these signals can also improve the possibility of detecting impending changes in the heart's condition so that prophylactic treatment can be started, e.g. to prevent the development of tachyarrhythmias, fibrillation in particular.
The autonomic nervous system innervates the heart by means of two sub-systems, the sympathetic nervous system and the parasympathetic nervous system respectively. The sub-systems will henceforth usually be referred to simply as the "sympathetic nerve" and "vagus nerve", unless otherwise specified. Increased signal activity in the sympathetic nerve increases heart activity, and increased signal activity in the vagus nerve reduces heart activity. Both systems normally balance each other when the body is at rest.
European Application 0 532 144 discloses a system for ANS control of a device for heart therapy. The device can also include a nerve stimulator, in addition to a conventional device for electrical heart therapy. In order to achieve a control signal related to the ANS, activity is detected in the sympathetic nerve by measurement of the regional, effective rise in impedance in the right ventricle. After the measurement signal is processed, the rise in impedance can be used as the control signal for the therapy device. Control could also be exercised through collaboration with one or more of the signals indicative of the body's work load, as noted above. In the device according to European Application 0 532 144, the activity of the sympathetic nerve or the activity of the nerve signal is indirectly sensed by measurement of this activity in the form of its effect on the heart via some appropriate parameter.
When the activity of the nerve signal is measured indirectly, the measurement becomes dependent on the ability of the measurement parameter to simulate the activity. If the patient suffers from some heart disease, which in particular may occur among patients in need of a heart therapy implant, this is not the case, and measurement in the heart will not then supply correct information about the activity of the nerve signal.
In addition to indirect measurement of the activity of the sympathetic nerve according to European Application 0 532 144, stimulation of the vagus nerve, more particularly its endocardiac ends, during impending tachyarrythmia has also been proposed (Max Schaldach "Electrotherapy of the Heart", 1992, Springer Verlag, Heidelberg, pp. 210-214).
In other electromedical therapy, e.g. the treatment of epilepsy, it is known to directly stimulate a nerve, more particularly the vagus nerve, by means of an implantable pulse generator. One such system having a helical electrode applied to the vagus nerve in the neck area is described in an article by Tarver et al.: "Clinical Experience with a Helical Bipolar Stimulating Lead", Pace, Vol. 15, October, Part II 1992, pp. 1545-1566. This system, however, only stimulates the nerve, and the pulse generator is controlled by means of an extracorporeally applied magnet.